Conventionally, there has been a vegetable cooking method described in JP 2008-111647 A (PTL1). In this vegetable cooking method, a vegetable such as spinach is exposed to a steam temperature atmosphere controlled to a specified steam temperature so that vitamin C remains in the vegetable in as large an amount as possible.
In the case where the food is spinach, insufficient heating would cause the lye to be too strong, making the spinach uneatable. On the other hand, vitamin C decreases due to heating, so that sufficient heating of the food would cause the quantity of vitamin C to decrease.